1. Field of the Invention
The present invention relates to an inorganic electroluminescent display device and a method of manufacturing the same, and more particularly, to an inorganic electroluminescent display device having improved light emission efficiency.
2. Discussion of the Background
Generally, an external light coupling efficiency ηex of an inorganic electroluminescent display device is given by:
                                          η            ex                    =                                    η              in                        ·                          η              out                                      ,                            [                  Equation          ⁢                                          ⁢          1                ]            
where, ηin and ηout denote an internal light coupling efficiency and an output coupling efficiency, respectively. The internal light coupling efficiency ηin is determined by self-elimination within each layer, and the output coupling efficiency ηout is determined by total reflection in each layer (i.e., failure to externally output light due to total reflection generated at an interface because the incident angle exceeds a critical angle when the light is incident from a medium having a higher refractive index to a medium having a lower refractive index). As described below, since the inorganic electroluminescent display device's luminescent layer has a higher refractive index than an insulation layer, the light coupling efficiency is usually determined by total reflection at the interface between the luminescent layer and the insulation layer.
In a conventional inorganic electroluminescent display device shown in FIG. 1, assuming that the light generated in the luminescent layer 140 passes through a first insulation layer 130, a first electrode 120, and a substrate 110 before being transmitted to the air, the output coupling efficiency ηout in consideration of the total reflection between each layer may be expressed as:
                                          1            2                    ⁢                                    (                                                N                  out                                                  N                  in                                            )                        2                          ,                            [                  Equation          ⁢                                          ⁢          2                ]            
where, N denotes an refractive index of each layer. Some examples of the indices of refraction N of the layers used in a typical inorganic electroluminescent display device are shown in the following Table 1.
TABLE 1Case ICase IICase IIINThicknessNThicknessNThickness2nd Insulation1.6200 nm1.9200 nm2.5200 nmLayerLuminescent2.5800 nm2.5800 nm2.5800 nmLayer1st Insulation1.6200 nm1.9200 nm2.5200 nmLayer1st Electrode1.9200 nm1.9200 nm1.9200 nmSubstrate1.5700 μm1.5700 μm1.5700 μm
In Table 1, the conventional luminescent layer typically comprises ZnS. Additionally, for the first and second insulation layers, SiO2 and Al2O3 are used in Case I, SiNx is used in Case II, and ZnS is used in Case III.
Then, for Case I, II, and III, ratios of the light incident to a layer to the light eliminated within the layer without being externally output are measured. Table 2 shows the results.
TABLE 2Case ICase IICase IIILuminescent Layer Mode83.3%77.1%77.6%1st Insulation Layer Mode5.3%11.9%1st Electrode Mode2.3% 7.9%Substrate Mode1.7%5.3%  4%Ratio of Light Output to External7.4%5.7%10.5%
In Table 2, the luminescent layer mode refers to a ray path caused by total reflection at an interface between the luminescent layer 140 and the first insulation layer 130. Similarly, the first insulation layer mode refers to a ray path caused by total reflection at an interface between the first insulation layer 130 and the first electrode 120, the first electrode mode refers to a ray path caused by total reflection at an interface between the first electrode 120 and the substrate 110, and the substrate mode refers to a ray path caused by total reflection at an interface between the substrate 110 and the air.
The numerical values of Table 2 were obtained using a finite difference time domain (FDTD) simulator, which accurately calculates Maxwell's equations and may ensure high reliability.
Referring to Table 2, which shows light coupling efficiency in consideration of optical properties of layers in an inorganic electroluminescent device, the amount of light transmitted to the first layer is 30% or less of the total light for Cases I and II, which is typical in the conventional art. This is caused by total reflection generated when light is incident from the luminescent layer 140 to the first insulation layer 130 (i.e., the luminescent layer mode). Also, for Case III, the light coupling efficiency is dominated by the total reflection at an interface between the first insulation layer 130 and the first electrode 120 (i.e. the 1st insulation layer mode).
Consequently, for all three cases, the amount of externally outputted light is reduced to about 10% of the original amount of generated light, as shown in the last row of Table 2. Therefore, the optical loss and luminance reduction in a corresponding display device is significant.
There have been several attempts to solve this optical loss and luminance reduction problem. For example, a supply voltage may be increased. While this may improve luminance, it adds complications for a driver IC. Also, increasing the supply voltage decreases the lifespan of main components and increases power consumption. Therefore, techniques have been proposed to provide improved luminance without increasing the driving voltage or with a decreased driving voltage.
For example, Japanese patent application publication No. 9-73983 discloses an electroluminescent display device including a prism lens sheet comprising an acrylic resin, in which a plurality of prisms have length-directional axes that are in parallel with one another. In this case, light incident on an interface between a transparent substrate and the air at an angle exceeding a critical angle experiences less total reflection due to the prism lens, where the incident angle is made less than the critical angle at each side. Also, luminance is increased in a predetermined direction by outputting light to the corresponding direction. However, according to this technique, optical loss may still be caused by reflection in the prism lens. Also, sharpness may be reduced due to overlapped images. Additionally, since most of the light that is not externally outputted due to total reflection is generated in the interface between the luminescent layer and the insulation layer, improvement of the light coupling efficiency may be slight.
Japanese patent publication No. 11-283751 discloses an organic electroluminescent display device having a diffraction grid on a reflection electrode. Here, the effect of total reflection may be reduced by using a diffraction grid structure formed on the reflection electrode so that light that has experienced total reflection at an interface may be incident to the interface at an angle smaller than a critical angle. However, though this technique may be effective when applied to an organic electroluminescent display device in which most of the total reflection is generated at an interface between the substrate and the electrode, it is not effective when applied to an inorganic electroluminescent display device in which most of the total reflection is generated at an interface between the luminescent layer and the insulation layer.